A field sequential full-color display system that requires no color filter is characterized in that a backlight that flashes “red, green, and blue” in sequence is used. The frame time of ordinary CRTs and liquid crystal displays is 16.7 ms. However, the frame time of the field sequential full-color display system is 5.6 ms, and fast responsiveness is required for the field sequential full-color display system.
One indicator of the fast responsiveness is the sum of τd and τr. Here, τd is the decay response time of the liquid crystal, and τr is the rise response time of the liquid crystal. To achieve the fast responsiveness in the field sequential full color display system, it is desired that the sum of τd and τr is less than 1.5 ms.
Currently, in the marketplace, liquid crystal materials called nematic liquid crystals are commonly used for flat panel displays of TV sets, monitors, mobile phones, smart phones, tablet terminals, etc. However, the nematic liquid crystals have a slow response speed of from ten-odd milliseconds to several milliseconds, and it is therefore desired to improve the response speed. Since the response speed of a liquid crystal is largely influenced by the rotational viscosity γ1 of the liquid crystal and its elastic constants, it has been attempted to improve the response speed by developing novel compounds and optimizing their chemical composition, but the progress of the improvement has slowed. In contract, ferroelectric liquid crystals (FLCs) using smectic liquid crystals are capable of fast response on the order of several hundreds of microseconds. However, since the ferroelectric liquid crystals have only two states, i.e., bright and dark states, halftone display necessary for full-color display is not easily obtained, and an area coverage modulation method, for example, is used.
Among the FLCs, a polymer stabilized V-shaped-FLC (PSV-FLC) element composed of a mixture of an FLC and a monomer includes a fine polymer network formed in the ferroelectric liquid crystal and not only has fast responsiveness, which is a feature of the FLC, but also is capable of halftone display. Moreover, the PSV-FLC shows improved impact resistance as compared with conventional FLCs (see, for example, PTL 1).
In a composite material of a nematic liquid crystal and a polymer, when a polymerizable compound is added to the nematic liquid crystal medium in an amount of 70% by mass or more, a fast response on the order to several tens of microseconds is obtained. However, the driving voltage of the element exceeds about 80 V, and the element is not suitable for practical use. Moreover, the effective birefringence of the element is lower than that of the liquid crystal used by at least one order of magnitude, and this causes a reduction in transmittance of the element. In previously proposed PS (polymer-stabilized) and PSA (polymer-sustained alignment) displays (see, for example, PTL 2 to PTL 6), at least one polymerizable compound is added to a liquid crystal medium in an amount of 0.3% by mass or more and less than 1% by mass, and then the polymerizable compound is subjected to ultraviolet photopolymerization while a voltage is applied or no voltage is applied. In this case, fine protrusions obtained by cross-linking or polymerization are famed at the interface between the liquid crystal medium and a glass substrate to thereby induce mainly a pretilt. However, there is room for improvement in terms of fast responsiveness of these devices. In particular, to increase the rise rate of a liquid crystal display device to achieve fast response, various techniques have been practically used such as reducing the viscosity of the liquid crystal composition, increasing its dielectric constant, reducing its elastic constants, imparting a pretilt angle, and improving a driving method such as an overdrive method. However, as for the decay rate, no effective technique other than reducing the viscosity of the liquid crystal composition has been found at present, and there is a need for improvement in the decay rate.